Polarization or variations in polarization is a major concern for optical applications such as telecommunication systems, fiber optics, optical sensors, optical amplifiers and optical sources, all of which may have performance characteristics that are dependent on the polarization of the light. Polarization dependent properties of these devices may include gain, insertion loss, spectral response, etc. For example, in optical fiber amplifiers the amount of gain obtained is of great importance. However, Raman amplifiers and Erbium Doped Fiber Amplifiers (EDFAs), have polarization dependent gain where the amount of gain varies as a function of the degree of polarization. The degree of polarization is the ratio of the power of polarized light to the power of the total light, ranging from ‘1’ (fully polarized light) to ‘0’ (unpolarized light). Raman amplifiers have inherently nonlinear amplification, meaning the amount of gain achieved by the amplifier depends on the polarization of the signal being transmitted. Variations in the polarization cause variations in the gain which provides an unsatisfactory response.
In EDFAs, gain in the polarization parallel to the signal is less than the gain in the orthogonal polarization state. Hence, unwanted light noise, also called amplified spontaneous emission (ASE) energy, in the orthogonal polarization state receives more gain than the signal, degrading the signal-to-noise ratio. Though this polarization dependent gain may be small (e.g., approximately 0.1 dB for an EDFA), the signal-to-noise ratio impairment can build up in systems containing cascaded amplifiers, and, thus, can adversely affect the overall signal transmission. Therefore, there is a need to either maintain the polarization (if systems are affected by changes in polarization) or completely depolarize the light.
Many proposals have addressed these issues. For example, polarization maintaining fibers (PMFs) are used to maintain linear polarization when the input electric field is aligned with the principle axis of the fiber. However, this technique only addresses variations in polarization, and not necessarily the degree of polarization. PMFs are only applicable to highly, linearly polarized light and require careful alignment with the optical axes of the fiber, and with each other if the PMFs are cascaded. PMFs are also costly; making long distance communications expensive.
Another solution is to depolarize the light. The state of polarization of the light is changed randomly such that the overall polarization over a given period of time can be considered to be depolarized. There are both active and passive methods of depolarization. Active methods induce modulation of a waveguide's optical properties. For example, an active method may involve modulating the refractive index or birefringence of a waveguide to alter the state of polarization using acoustic or electric waves. By cycling the refractive index over a period of time, no particular state of polarization dominates. However, the light is only considered depolarized when averaged over a period of time, but maintains a high degree of polarization during narrow time intervals. This short term polarization is a problem for high speed optical devices. In addition, active methods require power supplies and drive circuitry thereby adding cost and size to the system.
Passive depolarization methods, on the other hand, are cheaper and easier to implement than active methods. Passive methods include Lyot filters and fiber recirculating loops. Lyot filters may employ two strongly birefringent plates with their principal axes rotated 45 degrees relative to each other. Different wavelengths within the polarized light experience different amounts of retardation and hence a different state of polarization. In place of the plates, birefringent (polarization-maintaining) fibers of different lengths may be spliced together after rotating their principal axes by 45 degrees with respect to each other. However, these Lyot filters have a high component cost and are limited by the linewidth of light they can depolarize, making them inefficient for narrowband sources (as often used in telecommunication systems). Though the birefringent fibers may be used for narrowband sources, these require a long length of polarization maintaining fibers thereby adding cost and size to the system.
Fiber recirculating loops delay a portion of the polarized light and couple the recirculated light back into the fiber incoherently. The birefringence of the optical fiber alters the state of polarization. Fiber recirculating loops necessarily require a loop length much larger than the coherence length of the input light and narrowband sources inherently have a large coherence length. In addition, if the bend in the loop is too tight, the fiber becomes lossy. Therefore cascaded loops are often required in order to sufficiently depolarize the light. With all these considerations, fiber recirculating loops tend to have a large footprint which increases their implementation costs.